A flow cytometer used in medical and biological fields includes a fluorescence detection device that receives fluorescence emitted from a fluorochrome attached to a measurement object by irradiation with a laser beam to identify the type of the measurement object.
A laser beam used in such a fluorescence detection device generally has a Gaussian intensity distribution, and therefore its light intensity is high at the center but decreases toward the edge. In order to accurately measure fluorescence with high resolution by a fluorescence detection device, it is necessary to irradiate a measurement object with a laser beam at a uniform intensity. Therefore, when a laser beam having a Gaussian intensity distribution is used, the central part of the laser beam where the light intensity is substantially uniform is used to irradiate a measurement object. However, when passing through a predetermined measurement point in a flow channel, a measurement object flowing through the flow channel is displaced from the center of the measurement point. Therefore, attempts have been made to minimize the amount of displacement of a measurement object from the center of a measurement point. Further, attempts to expand a portion of a laser beam having a Gaussian intensity distribution where the light intensity is uniform have been made by increasing the width of the laser beam.
The online article “Introduction to Principles of FCM-VI,” which could be found at http://www.bc-cytometry.com/FCM/fcmprinciple—6-6.html on Dec. 6, 2007, discloses a flow cytometer and describes that the flow cytometer needs to satisfy the following requirements to achieve high measurement resolution: stabilization of the light intensity of a laser beam; reduction in the diameter of a stream of a sheath liquid allowed to flow together with a measurement object; and formation of a laminar flow of the sheath liquid.
However, high measurement resolution and efficient use of a laser beam cannot be achieved only by satisfying the above requirements, that is, stabilization of the light intensity of a laser beam, reduction in the diameter of a stream of a sheath liquid, and formation of a laminar flow of the sheath liquid.
For example, a part of a laser beam having a uniform light intensity and used for measuring a measurement object can be expanded by increasing the width of the laser beam, but an outer portion of the laser beam is not used to irradiate the measurement object, which is disadvantageous from the viewpoint of efficient use of a laser beam.